1. Field of the Invention
The present invention relates to an image forming apparatus, an image formation system and a fixing device.
2. Description of Related Art
In general, an electrophotographic image forming apparatus (such as a printer, a copy machine, and a fax machine) is configured to irradiate (expose) a charged photoconductor with (to) laser light based on image data to form an electrostatic latent image on the surface of the photoconductor. The electrostatic latent image is then visualized by supplying toner from a developing device to the photoconductor (image carrier) on which the electrostatic latent image is formed, whereby a toner image is formed. Further, the toner image is directly or indirectly transferred to the sheet, and thereafter fixed through heating and pressing at a fixing nip of a heating member (for example, a heating roller) and a pressing member (for example, a pressure roller), thereby forming an image on the sheet.
Conventionally, image formation systems have been practically used in which a sheet feeding apparatus that feeds a continuous sheet such as continuous roll paper and folded paper is connected at the preceding side of the image forming apparatus, and a winding apparatus that winds up the sheet on which an image has been formed by the image forming apparatus is connected at the succeeding side of the image forming apparatus.
At a fixing section (the part for fixation by heating and pressing the sheet on which a toner image is transferred) of the above-mentioned image forming apparatus, a sheet is present between the heating member and the pressing member even in a non-image formation period such as a standby period and a warming-up period which are not included in the image formation period. The sheet does not make contact with the heating member during the non-image formation period when the image forming apparatus has a member for separating the heating member and the pressing member, but may be deformed under the influence of the heat of the heating member since the sheet is stopped. In general, the heating member is configured to be rotatable even during the non-image formation period. As such, when the sheet is deformed under the influence of the heat of the heating member, the deformed part may make contact with the heating member and damage the heating member, and this damage may result in the damage on the image. In addition, sheet winding jam in which the heating member draws the sheet may be caused.
As described, in image forming apparatuses, the sheet may be deformed under the influence of the heat of the heating member during the non-image formation period, and when the sheet is deformed, the heating member may be damaged, or the sheet winding jam in which the heating member draws the sheet may be caused.
To solve such a problem, a method has been devised in which the heat is prevented from concentrating at one point on the sheet by moving the sheet during the non-image formation period. Such a technique is disclosed in Japanese Patent Application Laid-Open No. 2008-233770 and Japanese Patent Application Laid-Open No. 2007-041370, for example.
Japanese Patent Application Laid-Open No. 2008-233770 discloses a technique in which a conveyance roller pair for fixation provided on the downstream side in the sheet conveyance direction is driven/stopped in the sheet stopping period in which printing is not performed so as to convey the sheet in the conveyance direction at given intervals, whereby discoloration and deformation of the sheet due to heat in the fixation step is prevented.
Japanese Patent Application Laid-Open No. 2007-041370 discloses a technique in which a continuous conveyance mode for continuously conveying the sheet to the downstream side of the conveyance path, a conveyance stopping mode for stopping the sheet in the inserted state on the conveyance path, and a reciprocation mode for reciprocating the sheet on the conveyance path such that a certain part of the sheet is prevented from making contact with a certain member in a stopping state are provided. In this technique, the mode is switched to the continuous conveyance mode during image formation on the sheet, the mode is switched to the conveyance stopping mode when the image formation to the sheet is terminated, and the mode is switched to the reciprocation mode at a time point during the non-image formation period, whereby the amount of deformation of the sheet is reduced.
However, in the technique disclosed in Japanese Patent Application Laid-Open No. 2008-233770, if the sheet conveyance stopping period is fixed to a certain period, the sheet may disadvantageously be conveyed more than necessary and wasted, or deformation or discoloration of the sheet may disadvantageously be caused due to an excessively long sheet conveyance stopping period since the sheet conveyance stopping period for preventing heat deformation and discoloration of the sheet differs depending on the type of the sheet. This problem may be solved by changing the sheet conveyance stopping period in accordance with the type of the sheet; however, then the type has to be determined in advance, and consequently the setting is disadvantageously complicated.
In the technique disclosed in Japanese Patent Application Laid-Open No. 2007-041370, disadvantageously, reciprocation may be performed by an excessive length and consequently the sheet may be damaged and wasted, or the conveyance speed may be excessively slow and consequently the sheet may be deformed by heat, since the conditions such as the switching time from the stopping mode to the reciprocation mode for preventing the heat deformation, and the conveyance speed and the reciprocation length of the reciprocation mode are different depending on the type of the sheet.
As described, in the above-described two conventional techniques, the sheet is only simply moved, and the sheet is wasted by the amount of the movement if the sheet is needlessly moved.
Moreover, since the sheet has been damaged to some degree, the damaged part still cannot be used even after the sheet is needlessly moved.